An agricultural seeding machine, such as a row crop planter or grain drill, places seeds at a desired depth within a plurality of parallel seed trenches formed in soil. In the case of a row crop planter, a plurality of row crop units are typically ground driven using wheels, shafts, sprockets, transfer cases, chains and the like or powered by electric or hydraulic motors. Each row crop unit has a frame which is movably coupled with a tool bar. The frame may carry a main seed hopper, herbicide hopper and insecticide hopper. If a herbicide and insecticide are used, the metering mechanisms associated with dispensing the granular product into the seed trench are relatively simple. On the other hand, the mechanisms necessary to properly meter the seeds, and dispense the seeds at predetermined relative locations within the seed trench are relatively complicated.
The mechanisms associated with metering and placing the seeds generally can be divided into a seed metering system and a seed placement or delivery system which are in series communication with each other. The seed metering system receives the seeds in a bulk manner from the seed hopper carried by the planter frame or by the row unit. Different types of seed metering systems may be used, such as seed plates, finger plates, seed disks, belts, etc. In the case of a seed disk metering system, a seed disk is formed with a plurality of seed cells spaced about the periphery of the disk. Seeds are moved into the seed cells with one or more seeds in each seed cell depending upon the size and configuration of the seed cell. A vacuum or positive air pressure differential may be used in conjunction with the seed disk to assist in movement of the seeds into the seed cell. The seeds are singulated and discharged sequentially at a predetermined rate to the seed placement or delivery system.
The most common seed delivery system may be categorized as a gravity drop system. In the case of the gravity drop system, a seed tube has an inlet end which is positioned below the seed metering system. The singulated seeds from the seed metering system merely drop into the seed tube and fall via gravitational force from a discharge end thereof into the seed trench. Monitoring systems are commonly used to monitor the operation of the planter. Such systems typically employ a seed sensor attached to each seed tube to detect the passage of seed therethrough. Seed sensors are readily available from a number of sources including the Dickey-john Corporation, an example of which is described in U.S. Pat. No. 4,555,624. As described, the photo sensor consists of one or more light emitting devices and one or more photo-sensitive elements generally mounted on opposing sides of the seed tube. A seed passing through the tube momentarily partially obscures the light falling on one or more of the photo-sensitive elements, thereby producing a momentary change in the normal, or steady state signal level output from the sensor. Various problems are associated with this arrangement that effect the accuracy of the monitoring system. One problem is the dust and dirt that are drawn into the seed tube as the planter is operated. Another problem is the degree of ambient light in the seed tube. The greater the ambient light, the harder it is to detect seeds. To counter the effects of dirt, dust, and light, the sensors have been located near the top of the seed tube, away from the lower outlet opening. This however, allows a greater opportunity for variation to occur in the path traveled by the seed after it has past the sensor, affecting the accuracy of seed spacing determinations by the monitor.